Require at least php 5.3.8

Most versions of php less than 5.3.8 will cause some issues with the password compat library (see here: https://github.com/ircmaxell/password_compat#requirements). The easiest way to stop these potentially odd errors is just to require at least this minor version.

@dancryer this is related to the stange issue on the mailing list you are dealing with